1. Field of the Invention
The present invention relates to a digital modulator for converting a binary data to a channel code suitable for recording or transmission and a demodulator for converting the channel code back to the original binary data.
2. Description of the Prior Art
For the purpose of improvement in digital recording and digital communication, there has recently been utilized a digital modulator for performing a process known as group coding. The group coding divides data bits into groups and converts each group to a channel code. The group coding can improve the efficiency of coding, and allows for self-clocking.
As a digital modulation method of group coding, a (2,7) code method is disclosed in U.S. Pat. No. 3,689,899. This digital modulation method allows data bits composed of 2-, 3-, and 4-bit units to be converted into channel bits of 4-, 6-, and 8-bit units respectively. More specifically, the channel bits will contain 2 to 7 bits `0` between bits `1`.
Thus, the (2,7) code method is successfully used in the magnetic recording with the help of NRZI recording which involves a state transition at every channel bit `1`. Particularly, the interval of state transitions is represented by at least 1.5 data bits and becomes 1.5 times greater than that in the NRZI recording without code conversion, which will probably increase the density of recording 1.5 times. However, in optical recording, the NRZI recording is hardly effected due to a change in the duty ratio of the record signal. RZ recording is therefore employed in which a pulse mark having a particular width is given at each channel bit `1`. In this case, the mark length will be 0.75 times greater than the data bit length. The problem is that optical recording using the (2,7) code is disadvantageous for increasing the recording density as compared with magnetic recording. Also, as the number of channel bits is 2 times greater than that of data bits, the width of the channel bit becomes narrower and the jitter margin will be reduced.
U.S. Pat. No. 4,646,281 discloses a process named as four out of fifteen code (4/15 code) for demodulating a channel signal demodulated by group coding. In the 4/15 code method, an 8-bit data word is converted to a 15-bit code word in which the number of `1`s contained is limited to 4. Accordingly, a method of differential detection can be used when the code word is read out from a channel signal supplied from a transmission path. The differential detection is a procedure of detecting a code word in which, if `1` is high in the level and `0` is low, the channel signal is sampled in synchronism with the channel bits, and the first through fourth largest samples are each designated as `1` and the remaining samples are each designated as `0`. The advantage of the differential detection is that the noise margin will increase. However, as an 8-bit data word is converted to a 15-bit code word, the length of each bit decreases and thus, the jitter margin will be reduced. The reason for assigning the large number of bits, i.e. 15 bits, to the code word is to keep the number of bits `0` occurring between bits of `1` to more than one so that the minimum distance between transitions will not become small. Hence, the minimum transition interval is equivalent to 0.8 times the data bit length.
When an M-bit data word is converted to an N-bit code word in which N&gt;M, the jitter margin will increase as the ratio of N/M becomes smaller. For execution of the differential detection, a rule of conversion should be determined so that the number of `1`s contained in the code word is always a specific number i. The minimum of N in the code word converted from the M-bit data word under the abovementioned condition is determined naturally. Table 1 shows the relations among M, N and i.
TABLE 1 ______________________________________ M 4 5 6 7 8 9 10 N 6 7 8 10 11 12 13 i 3 3 or 4 4 4 to 6 4 to 6 5 to 7 5 to 8 ______________________________________
The numeral i is determined from EQU .sub.N C.sub.i .gtoreq.2.sup.M
where N is the minimum number corresponding to M. For example, if M is 8, the number of bits in the code word (N)is 11 and the jitter margin is greater than that of the 4/15 code. However, the minimum transition interval is 0.73 times the data bit length and shorter than that of the 4/15 code.